Influence of microwave application on convective drying: Effects on drying kinetics, and optical and mechanical properties of apple and strawberry

The influence of air temperature, microwave application and vacuum impregnation or pulsed vacuum osmotic dehydration, as pre-treatments undertaken prior to convective drying, on the drying kinetics of apple and strawberry has been studied. Furthermore, the effect of the above pre-treatments on the optical and mechanical characteristics of dried samples has also been investigated. Empirical equations (linear and page) were used to assess the effect of these factors on drying kinetics, with a good fit being observed between experimental data and model. The effect of microwave on the decrease in drying time was significantly greater than the effect of increasing the air temperature. A dehydrated product with less colour change and a more rigid and firmer structure was obtained at higher air temperature or when applying microwave. However, the higher temperatures during microwave treatment increased pigment degradation. The increase in the liquid phase volume occurring with pre-treatments prolonged the convective drying time process and also implied greater colour changes in the samples. Nevertheless, they enhance the resistance to deformation and fracture of the dehydrated product. (C) 2008 Elsevier Ltd. All rights reserved.